A thermistor is a semiconductor usually of a ceramic like material and comprised of a metallic oxide. Typically, the ceramic thermistor body is formed of a sintered mixture of manganese oxide, nickel oxide, ferric oxide, magnesium chromate or zinc chromate, or the like. A thermistor makes use of the resistive properties of semiconductors. Thermistors have a large negative temperature coefficient of resistivity such that as temperature increases, the resistance of the thermistor decreases.
A thermistor is connected into an electric circuit which utilizes the resistance of the thermistor in some manner. For effecting an electric connection to the thermistor, the thermistor has contacts attached to it. The contacts may take various forms, including contact areas or buttons on the surface of the thermistor, or bared metal conductors which pass through the thermistor and contact its ceramic material, including conductors soldered or otherwise affixed to the body of the thermistor, etc. The contacts of the thermistor are, in turn, connected by conductors to other circuit elements.
The ceramic bodies of thermistors are formed in many ways. One typical thermistor is in bead form, somewhat rounded in shape. It may be molded in that form or cut from a rod, etc. Another typical thermistor is in a wafer form and is multi-sided. The wafer usually is six sided and has two large area opposite surfaces and four narrower width peripheral sides defining the large opposite surfaces. A wafer thermistor may, for example, be cut from a larger sheet or other body of thermistor material or it may be molded. The ceramic material of the thermistor may be formed or cut in virtually any size. Various techniques for cutting, grinding or otherwise trimming thermistor bodies to a particular size are well known.
The resistance of a thermistor is in part determined by the volume of the semiconductor material of which it is comprised. As the thickness of the semiconductor material between the contacts in a particular thermistor is reduced, the resistance of the thermistor increases. More significant, however, is the observation that the smaller the thickness of the thermistor material, the greater is its response, in terms of change in its resistance, for any particular change in the temperature to which the thermistor is exposed. Thus, in a situation where very accurate rating of a thermistor is desired, it is beneficial to make the thickness of the element of semiconductor material in the thermistor as small as possible. This has led to production of small size bead or wafer thermistors, with a typical wafer thermistor having a semiconductor material thickness dimension of approximately 0.010 mm. and the semiconductor material having its larger surfaces with dimensions of 0.060 mm..times.0.060 mm.
One method of adjusting the resistance of the thermistor is by removing some of the semiconductor material between the thermistor contacts. Typically, however, the semiconductor material portions of the thermistor are mass produced in a uniform manner and removal of part of the semiconductor material of individual thermistors is difficult to accurately control without the expenditure of excessive amounts of time.
Another factor that determines the resistance of a thermistor is the surface area of the electric contacts of the thermistor which engage the conductors leading to the thermistor. It is the surface area of the contacts in actual contact with the semiconductor material of the thermistor that is important. Generally, the resistance of a thermistor, at constant temperature and pressure conditions, can be expressed by the formula R=.rho.t/A, wherein .rho. is the resistivity of the semiconductor material, t is the thickness dimension of the semiconductor material along the shortest distance between its two contacts and A is the surface area of contact material or of semiconductor material (depending upon the arrangement of the contacts) which is actually involved in the passage of current through the thermistor. (This is explained in fuller detail below in the detailed description.)
Where the contacts of the thermistor are comprised of bared sections of the conductors that pass through the thermistor, the surface areas of the thermistor contacts in actual engagement with the surface of the thermistor material is predetermined and invariable and essentially inaccessible for being changed. Hence, the resistance of this type of thermistor cannot be adjusted by changing the surface areas of the contacts on the thermistor semiconductor material.
In a thermistor wherein the metallic electric contacts are applied to the exterior of the semiconductor material, then the resistance of the thermistor can be adjusted by trimming away some of the surface area of the contacts of the thermistor from the semi-conductor material of the thermistor. It has been found that on a thermistor having only two metallic contacts, of silver or copper, for example, and wherein each contact is connected to a respective electric conductor in a circuit and the contacts are on opposite surfaces of the thermistor, that if the surface area on the semiconductor material of one or both contacts is trimmed by a particular percentage, then the resistance of the thermistor increases by the maximum percentage reduction of the surface area of one of the contcts. (Again, this is explained in greater detail below.) For example, if the surface area of at least one of the two contacts is reduced by 4%, then the resistance of the thermistor increases by 4%, i.e. it has a resistance of 4% more ohms than prior to the trimming. For example, a thermistor rated at 5,000 ohms will, after the trimming described just above, be rated at 5,200 ohms.
As noted above, thermistors are typically quite small in size. The surface area of their contacts on the surface of the semiconductor material of the thermistor is also small. Precise trimming of, for example, 1% or a fraction of a percent of the material of a thermistor contact is difficult.
Various techniques of trimming the contacts of thermistors are known. Obviously, a contact can be filed, sanded or otherwise ground away. Thermistors are so small and the change in their resistance that may be required is sometimes so small that rubbing a thermistor contact lightly once on a slightly roughened surface may trim off enough of the contact to change the rating of the thermistor to the desired extent. Manual or rubbing techniques for trimming thermistor contacts, as just described, are time consuming and can make thermistor manufacture and resistance rating quite expensive. There has, therefore, developed in combination with fine grinding or as an alternative thereto a technique of laser trimming, wherein a collimated laser beam is directed at a thermistor contact to burn away the desired amount of the contact.
Any technique of trimming a thermistor contact, e.g. fine grounding, laser trimming etc. operates within certain tolerance limits, whereby it is possible that a particular trimming procedure may trim slightly too little or too much of a contact, with an undesired discrepancy between the desired and actual resistance of a particular thermistor. A technique which permits trimming of a greater percentage of the surface area of a thermistor contact to bring about a relatively lesser percentage of change in the resistance of a thermistor would be desirable. With such a method, a slight error in the extent to which a thermistor contact is trimmed or the tolerances that trimming necessarily must be within will have a smaller effect on the final rating of the thermistor than they have with presently used trimming techniques.
I have been informed, although I have never seen the item, that there have been thermistors which simultaneously have two different resistance ratings. These thermistors have three contacts applied to their surfaces, rather than two. The third contact typically is considerably larger than the other two. If a wafer type thermistor, the two smaller contacts share one surface of the semiconductor material and the third contact covers virtually the entirety of another surface of the semiconductor material. Such a thermistor simultaneously has two different resistance ratings, depending upon which two of the three thermistor contacts are connected to the conductors of an electric circuit. If the conductors are attached to the two smaller size contacts on the one surface of the thermistor, the thermistor will have one resistance rating. If the conductors are instead connected to one of the two contacts on the one surface of the thermistor and to the larger size contact on the opposite surface of the thermistor, the thermistor will have a different resistance rating. This phenomenon occurs because the change in connection of the contacts changes the total surface area of the contacts and the width of the gap between the contacts, i.e. the thickness of the semiconductor material.
The applicability of three contact thermistors to more precise resistance rating of thermistors are not heretofore been recognized.
Obviously, when any of the factors affecting thermistor resistance change, then the resistance of the thermistor changes.